https://en.cppreference.com/w/cpp/compiler_support
And the bitcode used by Apple Store for application delivery isn't 100% like the LLVM bitcode.
Historically they've been very good about upstreaming. Sometimes it takes a while, but so far everything made it to the public repo.
It's quite common for long standing forks to exist, where changes get pushed upstream from the fork, and the fork gets periodically re-cut.
It looks to me like it’s the exact opposite, based solely on your link that is. Based on your link, they lag _behind_ upstream (vs being _ahead_ of upstream as you indicated) and don’t offer the same features, without a delay anyway, in “Apple Clang” that are in “Clang”.
We live in a post-FSF world, I think people are taking for granted the bean counters playing nice with open source software.
Why is this a reason for optimism? Because it shifts the outlet for innovation elsewhere. Costs of manufacturing will drop as competition in the fab space increases. I think RISC-V is a sign of things to come. There is great potential for open and amateur hardware.